1. Field of the Invention
The present invention relates generally to rotary bits for drilling subterranean formations. More specifically, the invention relates to fixed cutter, or so-called xe2x80x9cdragxe2x80x9d bits, employing superabrasive cutters exhibiting continuously varying cutter backrake angles along different locations or zones on the face of the bit, the variations being tailored to improve the transition between portions of the bit which may contain different cutter backrake angles as well as optimize the performance of the drill bit.
2. State of the Art
Conventional rotary-type earth-boring drill bits typically include cutting elements, or xe2x80x9ccutters,xe2x80x9d arranged thereon so as to facilitate the cutting away of a subterranean formation in a desired manner. Cutters, typically including polycrystalline diamond compacts (PDCs), are oriented in cutter pockets of the bit, which are oriented so as to protect the cutter and provide clearance at the trailing edge of the cutter as it moves axially while drilling. The angle at which a cutting face of a cutter is oriented relative to a wall of a bore hole being formed is referred to as xe2x80x9crake.xe2x80x9d If the angle between a bore hole surface and a cutter face is 90xc2x0, the rake is said to be neutral, or zero degrees. If the angle between the cutting face of a cutter and the adjacent surface of the bore hole being formed is less than 90xc2x0, the rake angle is negative, and is typically termed xe2x80x9cbackrake.xe2x80x9d The amount of backrake is equal to the angle the cutting face of the cutter is tilted from the neutral rake position. For example, a cutter oriented with its cutting face at a 70xc2x0 angle to the adjacent surface of the bore hole being formed has a 20xc2x0 backrake (90xc2x0xe2x88x9270xc2x0=20xc2x0). When the rake angle between the cutting face of a cutter and the adjacent bore hole surface is greater than 90xc2x0, the cutter is oriented with a positive, or aggressive, rake angle, or a xe2x80x9cfrontrake,xe2x80x9d which is measured in a similar manner to that in which backrake is measured.
Recent laboratory testing and modeling have demonstrated that cutter backrake angles may affect drilling performance characteristics. Specifically, increasing the backrake angle of a cutter appears to improve drilling performance after the cutter begins to wear. The wear flat of a cutter oriented at a larger backrake angle is smaller than the wear flat of a cutter oriented at a smaller (i.e., closer to neutral) backrake angle for a given amount of diamond volume removed. This means that as the diamond begins to wear away from the cutter, cutters oriented at larger backrake angles have smaller xe2x80x9cflatxe2x80x9d areas than do cutters oriented at smaller backrake angles. Smaller wear flats on cutters essentially provide a more effective cutting geometry. A sharp cutter (i.e., small wear flat) contacts a formation with less area and the same amount of force, thereby inducing larger stresses in the formation, increasing cutting efficiency. In addition, it has been found that orienting cutters to have larger backrake angles does not detrimentally affect the performance of the bit as cutter wear increases. Moreover, cutters that are oriented to have larger backrake angles typically provide better impact resistance than cutters that are oriented to have smaller backrake angles.
Although the aforementioned increased impact resistance and advantageous wear flat behavior is beneficial, the detriment to large backrake angles is that more weight on bit (WOB) is required to drill at a given rate of penetration (ROP). Therefore, generally, an all-encompassing increase in cutter backrake angles may cause the drill bit to require such a great WOB so as to render the bit undrillable.
Cutter rake not only affects the relationship between the ROP and the WOB but also determines the aggressiveness of the bit. Thus, the rakes of the cutters on a drag bit can affect the performance and drilling characteristics of the bit. The cutters on many drag bits are oriented so as to be backraked due to the increased fracture resistance of cutters with relatively large backrakes.
Current PDC drag bit design typically includes cutters oriented at different backrake angles depending upon their locations upon the bit. For example, cutters that are located within about a third of the bit radius from the bit""s longitudinal axis are typically oriented with nominal 15xc2x0 backrake angles. Cutters located in the shoulder area of the bit are oriented with backrake angles of about 20xc2x0. Cutters that are positioned near the gage section of the bit are typically oriented so as to have even higher backrake angles, for instance, about 30xc2x0. This discontinuous change in cutter backrake angle abruptly changes cutter behavior and performance between each area of the bit. This discontinuity may be exaggerated by the effective rake angles of the cutters.
Each cutter located on a bit crown at a given radial distance from the longitudinal axis of the bit will traverse a helical path upon rotation of the bit. The geometry (pitch) of the helical path is determined by the ROP of the bit (i.e., the rate at which the bit drills into a formation) and the rotational speed of the bit. Mathematically, it can be shown that the helical angle traversed by a cutter relative to a horizontal plane (i.e., a plane normal to the longitudinal axis of the bit) depends upon the distance the cutter is spaced apart from the longitudinal axis of the bit. For a given ROP and rotary speed, cutters located closer to the longitudinal axis have greater helical angles than those of cutters positioned greater distances from the longitudinal axis of the bit. Essentially, the greatest change in helical angles occurs for cutters positioned about 1xc2xd inches to about 2 inches from the bit""s longitudinal axis. In this region, the helical angles of the cutters during rotation of the bit vary from near 90xc2x0 for cutters nearest the longitudinal axis of the bit to about 7xc2x0 for cutters positioned about 2 inches from the longitudinal axis. The change in helical angle for cutters spaced about 2 inches from the longitudinal axis up to the bit gage is relatively small.
Effective cutter backrake is the angle between the cutter and the formation after correcting for the aforementioned helical angle during drilling (i.e., subtracting the helical angle of a cutter during drilling from the rake angle of the cutter). Since cutters may be at different radial locations, their cutting speeds will vary linearly with their radial position. This phenomenon of variance in xe2x80x9ceffective rakexe2x80x9d of a cutter with radial location, bit rotational speed, and ROP is known in the art and a more detailed discussion thereof may be found in U.S. Pat. No. 5,377,773, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein in its entirety by this reference.
Planar state of the art PDCs, as well as thermally stable products (TSPs) and other known types of cutters, are typically set at a given backrake angle on the bit face to enhance their ability to withstand axial loading of the bit, which is caused predominantly by the downward force applied to the bit during drilling, WOB. By comparing the effective backrake of a cutter, it is easy to see that cutters positioned within about 2 inches of the longitudinal axis of a bit are angled more aggressively than more distantly positioned cutters with the same or similar actual backrake angles.
As a result of the different effective rake angles of cutters that are oriented on a bit so as to have the same actual rake angles, these cutters wear differently, depending upon their radial distances from the longitudinal axis of the bit. Attempts have been made to correct for this problem through cutter redundancy, but the effectiveness of cutter redundancies is limited by the number of blades on the bit and by space constraints.
U.S. Pat No. 5,979,576 to Hansen et al. (hereinafter xe2x80x9cHansenxe2x80x9d), assigned to the assignee of the present invention, discloses anti-whirl drag bits with xe2x80x9cflankxe2x80x9d cutters placed in a so-called xe2x80x9ccutter-devoid zonexe2x80x9d at or near the gage area thereof. Typically, a bearing pad would be positioned on the bit in this region, and would accept the imbalance force, thereby keeping the bit stable. Instead, it is proposed in Hansen to place cutters located within the normally cutter-devoid area at a lesser height from the bit profile than other cutters and at positive, neutral, or negative rake angles. These cutters only engage the formation when the cutting zone cutters dull and the bit has a reduced tendency to whirl, or when the cutting zone cutters achieve relatively high depths of cut, such as when reaming or under high rates of penetration. Under high depths of cut, these cutters engage the formation and prevent damage to the bearing zone and thereby extend the life of the anti-whirl drag bit. While Hansen discloses flank cutters oriented at specific angles, Hansen does not disclose orienting the flank cutters on a bit at different rake angles from one another.
U.S. Pat No. 5,549,171 to Mensa-Wilmot et al. discloses drag bits with sets of cutters which are generally spaced the same radial distance from the longitudinal axis of the bit position but have differing backrakes. This may be accomplished by placing cutters with different backrakes onto different blades of the drag bit. Each set of cutters includes cutters oriented at the same rake angles. The cutters of different sets on a single blade may each have the same rake angles, or longitudinally adjacent sets of cutters offset, with a single blade of the bit including cutters oriented at different rake angles. The different rake angles of the cutters on each blade are not, however, angles that vary continuously (i.e., increase or decrease) along the height of the drag bit or with various radial distances from a longitudinal axis of the drag bit.
U.S. Pat No. 5,314,033 to Tibbitts (hereinafter xe2x80x9cTibbittsxe2x80x9d), assigned to the assignee of the present invention, discloses the use of xe2x80x9cpositivexe2x80x9d-raked cutters in combination with negative or neutral rake cutters in such a manner that the cutters work cooperatively with one another. Effectively positive raked cutters are disclosed as aggressively initiating the cutting of the formation, whereas effectively negative raked cutters are disclosed as skating or riding on the formation. This causes two vastly different cutting mechanisms to coincide on the drill bit, with sudden changes at the coincident boundary between areas with different effective backrakes. Tibbitts does not, however, disclose a bit that includes regions on the face thereof with cutters oriented at different, continuously varying positive or negative rake angles.
The inventors are not aware of any art that discloses drag bits with fixed cutters at a particular region of the bit that are oriented so as to have different, continuously varied rake angles.
The present invention includes rotary drag bits with fixed cutters having substantially continuously varied rake angles corresponding to the locations of the cutters relative to the longitudinal axis of the drag bit. As used herein, the term xe2x80x9crakexe2x80x9d refers to the radial angle of a cutting face of a cutter relative to a reference line perpendicular to a surface of a formation being drilled, as described previously herein.
In one embodiment of a drag bit incorporating teachings of the present invention, cutters are oriented to have rake angles that increase proportionately with an increase of the radial distance of cutter locations from the longitudinal axis of the drag bit.
In another embodiment of the present invention, a drag bit includes a face with a plurality of radially separate cutter zones or regions thereon. Each cutter zone includes a number of cutters oriented so as to have the same backrake angle. The cutters of one zone on the face of the drag bit will, however, be oriented to have rake angles that differ from the cutters located within the one or more other zones on the face of the drag bit. In regions where two adjacent zones border one another, cutters adjacent to the border are oriented so as to have rake angles that provide a smooth transition between the rake angles of cutters in each of the adjacent zones. In addition, a given zone or region may include a sequence of cutters having increasing, decreasing, increasing then decreasing, decreasing then increasing, or cyclical variations in rake angles.
Another embodiment of drag bit according to the present invention also includes fixed cutters within at least a region or zone over the bit face which are oriented to have rake angles that vary continuously, but not necessarily proportionately to the radial distance of each of the cutters from the longitudinal axis of the drag bit. Rather, other factors, such as the longitudinal location or the angle of the helical path of each cutter, may be taken into account in determining the rake angle at which each of the cutters is oriented.
A drag bit incorporating teachings of the present invention may include at least three cutters oriented so as to have rake angles that increase or decrease sequentially based upon the relative radial locations of the cutters on the drag bit, the relative longitudinal positions of the cutters on the drag bit, or the relative positions of the cutters on a blade of the drag bit.
The rake angles of cutters on drag bits of the present invention may take into account the angle of the helical path each cutter travels during rotation of the drag bit. The angle of the helical path may be accounted for by continuously varying the effective rake angles of the cutters depending upon their position on the drag bit so as to counteract the effective rakes of the cutters caused by the angles of the helical paths of the cutters.
It is also contemplated that the rake angles of different cutters may be varied in response to bit performance factors. By way of example, weight on bit as a function of torque data may be analyzed and cutters within at least one region on the face of a drag bit may be oriented at rake angles that are continuously varied so as to provide a torque response as a function of weight on bit. As another example, the rake angles at which different cutters within a particular region of a face of a drag bit are oriented may be selected in response to bit stability data. Directional drilling criteria may also be used to determine the different, continuously varied rake angles of cutters within a particular region on a face of a drag bit. Other examples of factors that may be considered to determine the specific, continuously varied rake angle of different cutters on a face of a drag bit include, but are not limited to, wear characteristics, formation type, cutter loading, rock stresses, filtration and filtration gradients versus design depth of cut in permeable rocks, and thermal loading.